Quantum-dot triggered photon and triggered photon pair

ABSTRACT

The present invention provides a device and method for producing triggered single photons and triggered pairs of polarization-entangled photons. A light source delivers a pulse to a photon emitter and generates pairs of electrons and holes to emit photons. The light source includes means to tune a pulse wavelength to an excited state-absorption resonance of the photon emitter. The light source could also include means to selectively choose a polarization to create pairs of electrons and holes of a particular spin. A filter isolates the last and single photon. Optionally a micro-cavity is included to direct the emitted photons and couple to one or more optical elements. When the device or method is used to produce triggered pairs of polarization-entangled photons it works almost the same as for the single photons, except for modifications to the way the light source excites the photon emitter and how emission filtering is performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is cross-referenced to and claims priority from U.S.Provisional application 60/256,006 filed Dec. 15, 2000, which is herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to quantum communication andcomputation. More particularly, the present invention relates to adevice and method for producing triggered single photons and triggeredpairs of polarization-entangled photons using a quantum dot.

BACKGROUND

Streams of single photons, or single pairs of polarization-entangledphotons, arriving within known time intervals have potentialapplications in the new fields of quantum communications and quantumcomputing. Most significantly, the recently demonstrated scheme ofquantum cryptography involves encoding information on the polarizationof a single photon. Security from eavesdropping is provided by the factthat one cannot measure the polarization of a single photon withoutaltering it. Large expenditures will soon be directed towards researchon quantum cryptography and efficient single photon sources.

Several protocols for quantum cryptography have been proposed, of which“BB84” in reference “C. H. Bennett and G. Brassard. Proceedings of IEEEInternational Conference on Computers, Systems, and Signal Processing,Bangalore, India IEEE, New York, 1984, pp. 175-179, 1984” and “Ekert” inreference “A. K. Ekert. Quantum cryptography based on Bell's theorem.Physical Review Letters, 67(6), 661-663, 1991” are two popular examples.In the BB84 protocol, a stream of single photons is needed. Thepresently available source, a highly attenuated laser, leaves thearrival photon number random, and thus requires a much lower datatransmission rate to avoid two-photon events, than would be possiblewith a regulated photon source. One of the most successful methods sofar to generate single photons is single-molecule fluorescence. Amolecule is excited by a laser pulse, and emits a single photon inresponse. This approach suffers from two difficulties, rapidphoto-bleaching (destruction of optical activity) of the molecules, andlimited collection efficiency of the emitted photons. Another approachis fluorescence from single color center in diamond crystals. However,in this approach the emitted photons have a very large variation inwavelength. In the Ekert protocol, a stream of polarization-entangledphoton pairs is needed. The most practical existing entangled photonsource, parametric down-conversion, produces a number of photon pairsaccording to a Poisson distribution, rather than deterministicallyproducing exactly one photon pair. A source of single pairs ofpolarization-entangled photons would be beneficial to this scheme. A fewother applications for such photon sources are possible, including arandom number generator, a light intensity standard, fundamental testsof quantum mechanics, quantum teleportation and quantum computation.

Accordingly, there is a need to develop a device and method that iscapable of providing both triggered single photons of definitepolarization, and triggered pairs of polarization-entangled photons. Inaddition, there is a need to develop a device and method that does notsuffer from photo bleaching, and allows a high collection efficiency.

SUMMARY OF THE INVENTION

The present invention provides a device and method for producingtriggered single photons and triggered pairs of polarization-entangledphotons.

The device and method for producing triggered single photons includes aphoton emitter, a light source and a filter. The light source delivers apulse to the photon emitter and generates pairs of electrons and holesinside the photon emitter to emit photons. The photon emitter could forinstance be a semiconductor quantum dot. The light source includes meansto tune a pulse wavelength to an excited state-absorption resonance ofthe photon emitter. The light source could also include means toselectively choose a polarization to create pairs of electrons and holesof a particular spin. The light source is intense enough that at leastone of the pairs of electrons and holes is generated with a highprobability for a pulse. The light source is usually a pulsed laser. Thefilter isolates the last and single photon from the emitted photons andis, for instance, an interference filter or a diffraction gratingmonochrometer. The filter includes means to distinguish betweenexcitonic and biexcitonic emissions lines. The filter also rejectsscattered light from the light source.

The device and method for producing triggered single photons optionallyincludes a micro-cavity to direct the emitted photons into a singlespatial mode and coupling single mode single photons to one or moreoptical elements such as an optical fiber. The micro-cavity includes asmall volume and a means for long photon storage time. The micro-cavityis, for instance, a micro-post distributed-Bragg-reflector cavity or amicro-sphere cavity.

When the device or method of the present invention is used to producetriggered pairs of polarization-entangled photons it works almost thesame as the single photon device and method as described above, exceptfor modifications to the way the light source excites the photon emitterand how emission filtering is performed. The device and method forproducing triggered pairs of polarization-entangled photons includes aphoton emitter; and a light source to deliver a pulse to the photonemitter to generate two electron-hole pairs with opposite spin insidethe photon emitter to emit two photons. The first of the two photons isat a biexcitonic wavelength and second of the two photons is at anexcitonic wavelength. The device and method for producing triggeredpairs of polarization-entangled photons optionally includes amicro-cavity to increase the collection efficiency of the emittedphotons and to direct the emitted photons into a single spatial mode andcoupling the single mode single photons to one or more optical elementssuch as an optical fiber.

Alternatively, the device and method producing triggered pairs ofpolarization-entangled photons according to the present invention couldinclude a photon emitter and a light source to deliver two pulses insuccession to the photon emitter. The first of the two pulses is tunedto a narrow absorption resonance to generate a first electron-hole pairin an excited state, and with a polarization of the light source chosento yield a definite spin, and second of the two pulses at a slightlylower energy is tuned to a narrow absorption resonance to generate asecond electron-hole pair in an excited state, and with a polarizationchosen to yield a spin opposite to that of the first electron-hole pair.

Another alternative device and method producing triggered pairs ofpolarization-entangled photons according to the present inventionincludes a photon emitter and a light source to deliver a pulse to thephoton emitter generating several electron-hole pairs of both spins. Inaddition, this device and method includes a filter to isolate only asingle-exciton emission line and narrowly accept only the emissions linecorresponding to two electron-hole pairs with opposite spins.

In view of that which is stated above, it is the objective of thepresent invention to provide a device and method for producing triggeredsingle photons.

It is another objective of the present invention to provide a device andmethod producing triggered pairs of polarization-entangled photons.

It is yet another objective of the present invention to tune a pulsewavelength of the light source to an excited state-absorption resonanceof the photon emitter.

It is still another objective of the present invention to include meansto selectively choose a polarization to create pairs of electrons andholes of a particular spin.

It is still another objective of the present invention to provide alight source that is intense enough that at least one of the pairs ofelectrons and holes is generated with a high probability for a pulse.

It is another objective of the present invention to include amicro-cavity to increase the collection efficiency of the emittedphotons and to direct the emitted photons into a single spatial mode andto couple the single mode single photons to one or more opticalelements.

Advantages of the present invention over the prior art are that thesystem enables one to generate single photons with narrow spectralline-widths, without photo-bleaching, and that the device may beincorporated into larger solid structures. Further advantages of thepresent invention over the prior art are that the system enables one togenerate single pairs of polarization-entangled photons, whereas themost practical existing source, spontaneous parametric down-conversion,can only generate a Poisson-distributed number of polarization-entangledphoton pairs.

BRIEF DESCRIPTION OF THE FIGURES

The objectives and advantages of the present invention will beunderstood by reading the following detailed description in conjunctionwith the drawings, in which:

FIG. 1 shows a single photon source according to the present invention;

FIG. 2 shows a single photon source with a polarizer according to thepresent invention;

FIG. 3 shows a single photon source with a polarizer and a gratingmonochrometer according to the present invention;

FIG. 4 shows a single photon source with a polarizer and micro-postcavity according to the present invention;

FIG. 5 shows a single photon source with a polarizer and a micro-sphereresonator according to the present invention;

FIG. 6 a polarization-entangled photon pair source; and

FIG. 7 a polarization-entangled photon pair source using two separatecavity modes to spectrally select 1X and 2X lines.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics forthe purposes of illustration, anyone of ordinary skill in the art willreadily appreciate that many variations and alterations to the followingexemplary details are within the scope of the invention. Accordingly,the following preferred embodiment of the invention is set forth withoutany loss of generality to, and without imposing limitations upon, theclaimed invention.

The present invention provides a device and method for producingtriggered single photons and triggered pairs of polarization-entangledphotons. According to a preferred embodiment of the present invention,the device and method for producing triggered single photons andtriggered pairs of polarization-entangled photons include the followingcomponents: (i) a photon emitter, (ii) a light source, and (iii) aspectral filter. Optionally, the present invention includes amicro-cavity.

FIG. 1 shows a single photon device 100 for producing triggered singlephotons. Device 100 includes a photon emitter 110, which is preferablyan isolated semiconductor quantum dot. Electrons and holes confinedinside photon emitter 110 have discrete energy levels as in atoms. Thequantum dot is, for instance, but not limited to, a tiny island ofsmaller-bandgap semiconductor material 112 surrounded by alarger-bandgap matrix 114. Typical examples of photon emitter 110include an InGaAs region surrounded by GaAs, an InP region surrounded byGaInP, or a GaAs region surrounded by AlGaAs. The quantum dot could, forexample, also be an InAs self-assembled dot.

Light source 130 delivers pulse 140 and excites photon emitter 110.Light source 130 can be a light source with a spectrum in the nearinfrared range. Pulse 140 generates pairs of electrons and holes insidephoton emitter 110 to emit photons 150-1X, 150-2X, and 150-3X. Lightsource 130 is preferably a tunable, pulsed laser. Light source 130includes means to tune the pulse wavelength of pulse 140 to an excitedstate-absorption resonance of photon emitter 110. Another way of tuningphoton emitter 110 in an excited state-absorption resonance is, forinstance, by changing the temperature of the photon emitter 110.

Light source 130 should be intense enough that at least one of the pairsof electrons and holes is generated with a high probability for pulse140. The generated electrons and holes then rapidly relax down to thelowest unoccupied energy levels, and begin to recombine, emittingphotons 150-1X, 150-2X, and 150-3X. The emitted photon 150-1X, 150-2X,and 150-3X wavelengths reflect not only the unperturbed energy levels ofthe recombining electrons and holes, but also the number of otherelectron-hole pairs present inside of the dot at that time, due toelectrostatic interactions. Thus, the last electron-hole pair torecombine emits at a unique wavelength.

Spectral filter 160 isolates the last and single photon 170 from emittedphotons 150-1X, 150-2X, and 150-3X. Filter 160 has a resolution that isable to distinguish between excitonic (one electron and one hole) andbiexcitonic (two electrons and two holes) emissions lines, such as thatit can be tuned to the one-exciton line. For typical dots, these linesare about 1-2 nanometers apart in wavelength, requiring preferably ahigh-resolution spectral filter or an interference filter. Emittedphotons like 2X (150-2X), 3X (150-3X)etc. may be present depending onthe excitation properties of pulse 140. Filter 160 also rejectsscattered light from light source 130.

In addition, the device 100 in FIG. 1 includes a polarizer 210, creatingdevice 200 in FIG. 2, wherein one can choose the polarization of lightsource 130 to selectively create an electron-hole pairs of a particularspin. If this spin is preserved until the electron and hole recombine,then the emitted photon 220-1X will have a well-controlled polarization,an advantage for quantum cryptography. The last emitted photon, in casepolarizer 210 is used, is now a polarized last emitted photon 230. Inthis particular example, emitted photons like 2X (220-2X), 3X(220-3X)etc. may be present depending on the excitation properties ofpulse 140.

FIG. 3 shows an example of a grating monochometer 310 in single photondevice 300 that is used as filter to filter emitted photons 320-1X and320-2X and tune to a 1X line to emit the last photon 330. In thisparticular example, grating monochometer 310 includes an input slit 380projected on an output slit 390 by means of a lens 340, concave mirrors350, a diffraction grating 360 and a plane mirror 370.

FIG. 4 shows single photon device 400 to emit photon 410 with an exampleof a micro-cavity, which is optional, and used to increase theefficiency, with which the photons are collected, by directing as manyof the emitted photons as possible into a single spatial mode forsubsequent coupling into optical fibers or other optical element.Without such a micro-cavity, the photons are emitted in randomdirections, and most are lost. A convenient cavity useful for thisfunction is a micropost, distributed-Bragg-reflector (DBR) cavity asshown in FIG. 4 by 420. Alternating crystal layers 422 and 424 with twodifferent indices of refraction are grown on either side of the quantumdot 430 to form a high-quality cavity, and then a tiny (<1 micron) postis etched around dot 430, through the entire DBR structure.

Another example of a micro-cavity is a single photon device 500 with amicro-sphere resonator 510 to receive photon 520 as shown in FIG. 5 thatis coupled to an output coupler 530, or prism, to direct single photon540. In micro-sphere resonator 510 a tiny glass sphere is held close tothe dot. High quality “whispering-gallery” modes exist along the surfaceof the sphere, and light couples into and out of these modes throughevanescent fields. However, any cavity with small volume and long photonstorage time can potentially serve this purpose.

When the device or method of the present invention is used to producetriggered pairs of polarization-entangled photons it works almost thesame as the single photon device and method as described above, exceptfor modifications to the way the light source excites the photon emitterand how emission filtering is performed. When the device of the presentinvention is used to produce triggered pairs of polarization-entangledphotons as shown in FIG. 6 by 600, laser pulse 610 creates exactly twoelectron-hole pairs with opposite spins. The electron-hole pairs relaxdown to the ground state, and when they recombine, they emit two photons620 and 630, one of which emits at the biexcitonic wavelength and theother emits at the excitonic wavelength. If the selection rules arestrong, and if spin relaxation is slow compared to the radiativelifetime, then a pair of polarization-entangled photons is produced foreach laser pulse 610. FIG. 6 is an exemplary embodiment of apolarization-entangled photon pair device 600 emitting the last twophotons 620 and 630. In this case, spectral filter 640 is selected topass 1X and 2X lines from emitted photons 640-1X, 640-2X and 640-3X.

FIG. 7 shows another exemplary embodiment of a polarization-entangledphoton pair device 700 emitting the last two photons 710 and 720 usingtwo separate cavity modes to spectrally select 1X and 2X lines using amicropost micro-cavity 730 similar to 410 in FIG. 4. Micropostmicro-cavity 730 contains alternating crystal layers 732 and 734 withtwo different indices of refraction that are grown on either side of thequantum dot 740 to form a high-quality cavity, and then a tiny (<1micron) post is etched around dot 740, through the entire DBR structure.The present invention includes the case where a micro-cavity is used toimprove the collection efficiency of the emitted photons. A special caseof this is where one transverse spatial mode of the micro-cavity isresonant with the 1X line, and another transverse spatial mode isresonant with the 2X line. In this case, the 1X and 2X photons areemitted in different directions, and can be separated from each otherwithout the need for a beam-splitter.

Exciting two electron-hole pairs with opposite spins could beaccomplished by, for instance, two methods. One possible method uses twolaser pulses in rapid succession. The first laser pulse is tuned to anarrow absorption resonance to generate one electron-hole pair in anexcited state, and with the polarization chosen to yield a definitespin. After the electron-hole pair relaxes down to the ground state, asecond pulse at slightly different energy (to take into account thebiexcitonic effect) is tuned to a narrow absorption resonance togenerate another electron-hole pair in an excited state, and withpolarization chosen to yield a spin opposite to that of the first pair.The emission filter rejects scattered light from the excitation laser.

In an alternative method, several electron-hole pairs of both spins arecreated with a single laser pulse. When only two pairs are left, thereis roughly a fifty-percent chance that they will have opposite spins. Ifthey have same spins, then one of the electron-hole pairs must be in theexcited state, due to the Pauli exclusion principle. Thus, the spatialwavefunction will be different from that of the ground state, and thebiexcitonic energy shift will be slightly different. With high enoughfiltering resolution, these two cases can be distinguished for thesecond-to-last emitted photon. Thus, if filters are used to accept onlysingle-exciton emission and narrowly accept only the line correspondingto two electron-hole pairs with opposite spins, two photons withfifty-percent probability are received, which have entangledpolarizations. This method requires emission lines being narrow so thatmany different states of the dot can be spectrally resolved, includingthe same-spin and opposite-spin biexcitonic states, and also states withmore than two electron-hole pairs, which must be rejected while passingthe biexcitonic line. If a micro-cavity is used to improve collectionefficiency, both the excitonic and biexcitonic emission lines must be inresonance.

The present invention has now been described in accordance with severalexemplary embodiments, which are intended to be illustrative in allaspects, rather than restrictive. Thus, the present invention is capableof many variations in detailed implementation, which may be derived fromthe description contained herein by a person of ordinary skill in theart. All such variations are considered to be within the scope andspirit of the present invention as defined by the following claims andtheir legal equivalents.

1. A device for producing triggered single photons, comprising: a) aphoton emitter; b) a light source to deliver a pulse to said photonemitter generating pairs of electrons and holes inside said photonemitter to emit photons, wherein said light source comprises means totune a pulse wavelength to an excited state-absorption resonance of saidphoton emitter; and c) a filter to isolate the last and single photonfrom said emitted photons, said filter comprises means to distinguishbetween excitonic and biexcitonic emissions lines.
 2. The device ofclaim 1, wherein said light source further comprises means toselectively choose a polarization to create said pairs of electrons andholes of a particular spin.
 3. The device of claim 1, wherein said lightsource is intense enough that at least one of said pairs of electronsand holes is generated with a high probability for said pulse.
 4. Thedevice of claim 1, wherein said light source is a pulsed laser.
 5. Thedevice of claim 1, wherein said photon emitter is a semiconductorquantum dot.
 6. The device of claim 5, wherein said semiconductorquantum dot comprises a tiny island of smaller-bandgap semiconductormaterial surrounded by a larger-bandgap matrix.
 7. The device of claim6, wherein said semiconductor quantum dot is an InGaAs region surroundedby GaAs, an InP region surrounded by GaInP, or a GaAs region surroundedby AlGaAs.
 8. The device of claim 1, wherein said filter rejectsscattered light from said light source.
 9. The device of claim 1,wherein said filter is an interference filter or a diffraction gratingmonochrometer.
 10. The device of claim 1, further comprising amicro-cavity to direct said emitted photons into a single spatial modeand coupling said single mode single photons to one or more opticalelements.
 11. The device of claim 10, wherein said micro-cavitycomprises a small volume and a means for long photon storage time. 12.The device of claim 10, wherein said micro-cavity is a micro-postdistributed-Bragg-reflector cavity or a micro-sphere cavity.
 13. Amethod for generating triggered single photons, comprising the steps of:(a) providing a photon emitter; (b) delivering a pulse to said photonemitter with a light source to generate pairs of electrons and holesinside said photon emitter to emit photons, wherein said light sourcecomprises means to tune a pulse wavelength to an excitedstate-absorption resonance of said photon emitter; and (c) filteringsaid emitted photons with a filter to isolate the last and singlephoton, said filter comprises means to distinguish between excitonic andbiexcitonic emissions lines.
 14. The method of claim 13, wherein saidlight source further comprises means to selectively choose apolarization to create said pairs of electrons and holes of a particularspin.
 15. The method of claim 13, wherein said light source is intenseenough that at least one of said pairs of electrons and holes isgenerated with a high probability for said pulse.
 16. The method ofclaim 13, wherein said light source is a pulsed laser.
 17. The method ofclaim 13, wherein said photon emitter is a semiconductor quantum dot.18. The method of claim 17, wherein said semiconductor quantum dotcomprises a tiny island of smaller-bandgap semiconductor materialsurrounded by a larger-bandgap matrix.
 19. The method of claim 18,wherein said semiconductor quantum dot is an InGaAs region surrounded byGaAs, an InP region surrounded by GaInP, or a GaAs region surrounded byAlGaAs.
 20. The method of claim 13, wherein said filter rejectsscattered light from said light source.
 21. The method of claim 13,wherein said filter is an interference filter or a diffraction gratingmonochrometer.
 22. The method of claim 13, further comprising the stepof providing a micro-cavity to direct said emitted photons into a singlespatial mode and coupling said single mode single photons to one or moreoptical elements.
 23. The method of claim 22, wherein said micro-cavitycomprises a small volume and a means for long photon storage time. 24.The method of claim 22, wherein said micro-cavity is a micro-postdistributed-Bragg-reflector cavity or a micro-sphere cavity.
 25. Adevice for producing triggered pairs of polarization-entangled photons,comprising: (a) a photon emitter; and (b) a light source to deliver apulse to said photon emitter generating two electron-hole pairs withopposite spin inside said photon emitter to emit two photons, whereinfirst of said two photons is at a biexcitonic wavelength and second ofsaid two photons is at an excitonic wavelength.
 26. The device of claim25, wherein said light source is a pulsed laser.
 27. The device of claim25, wherein said photon emitter is a semiconductor quantum dot.
 28. Thedevice of claim 27, wherein said semiconductor quantum dot comprises atiny island of smaller-bandgap semiconductor material surrounded by alarger-bandgap matrix.
 29. The device of claim 28, wherein saidsemiconductor quantum dot is an InGaAs region surrounded by GaAs, an InPregion surrounded by GaInP, or a GaAs region surrounded by AlGaAs. 30.The device of claim 25, further comprising a micro-cavity to increasethe collection efficiency of said emitted photons and to direct saidemitted photons into a single spatial mode and coupling said single modesingle photons to one or more optical elements.
 31. The device of claim30, wherein said micro-cavity comprises a small volume and a means forlong photon storage time.
 32. The device of claim 30, wherein saidmicro-cavity is a micro-post distributed-Bragg-reflector cavity or amicro-sphere cavity.
 33. The device of claim 30, wherein saidmicro-cavity has a first spatial mode that is resonant with the 1X lineand a second spatial mode that is resonant with the 2X line.
 34. Amethod for producing triggered pairs of polarization-entangled photons,comprising the steps of: (a) providing a photon emitter; and (b)delivering a pulse to said photon emitter with a light source togenerate two electron-hole pairs with opposite spin inside said photonemitter to emit two photons, wherein first of said two photons is at abiexcitonic wavelength and second of said two photons is at an excitonicwavelength.
 35. The method of claim 34, wherein said light source is apulsed laser.
 36. The method of claim 34, wherein said photon emitter isa semiconductor quantum dot.
 37. The method of claim 36, wherein saidsemiconductor quantum dot comprises a tiny island of smaller-bandgapsemiconductor material surrounded by a larger-bandgap matrix.
 38. Themethod of claim 37, wherein said semiconductor quantum dot is an InGaAsregion surrounded by GaAs, an InP region surrounded by GaInP, or a GaAsregion surrounded by AlGaAs.
 39. The method of claim 34, furthercomprising the step of providing a micro-cavity to increase thecollection efficiency of said emitted photons and to direct said emittedphotons into a single spatial mode and coupling said single mode singlephotons to one or more optical elements.
 40. The method of claim 39,wherein said micro-cavity comprises a small volume and a means for longphoton storage time.
 41. The method of claim 39, wherein saidmicro-cavity is a micro-post distributed-Bragg-reflector cavity or amicro-sphere cavity.
 42. The method of claim 39, wherein saidmicro-cavity has a first spatial mode that is resonant with the 1X lineand a second spatial mode that is resonant with the 2X line.
 43. Adevice for producing triggered pairs of polarization-entangled photons,comprising: (a) a photon emitter; and (b) a light source to deliver twopulses in succession to said photon emitter, wherein first of said twopulses is tuned to a narrow absorption resonance to generate a firstelectron-hole pair in an excited state, and with a polarization of saidlight source chosen to yield a definite spin, and second of said twopulses at a slightly lower energy is tuned to a narrow absorptionresonance to generate a second electron-hole pair in an excited state,and with a polarization chosen to yield a spin opposite to that of saidfirst electron-hole pair.
 44. The device of claim 43, wherein said lightsource is a pulsed laser.
 45. The device of claim 43, wherein saidphoton emitter is a semiconductor quantum dot.
 46. The device of claim45, wherein said semiconductor quantum dot comprises a tiny island ofsmaller-bandgap semiconductor material surrounded by a larger-bandgapmatrix.
 47. The device of claim 46, wherein said semiconductor quantumdot is an InGaAs region surrounded by GaAs, an InP region surrounded byGaInP, or a GaAs region surrounded by AlGaAs.
 48. The device of claim43, further comprising the step of filtering to reject scattered lightfrom said light source.
 49. The device of claim 43, further comprising amicro-cavity to increase the collection efficiency of said emittedphotons and to direct said emitted photons into a single spatial modeand coupling said single mode single photons to one or more opticalelements.
 50. The device of claim 49, wherein said micro-cavitycomprises a small volume and a means for long photon storage time. 51.The device of claim 49, wherein said micro-cavity is a micro-postdistributed-Bragg-reflector cavity or a micro-sphere cavity.
 52. Thedevice of claim 49, wherein said micro-cavity has a first spatial modethat is resonant with the 1X line and a second spatial mode that isresonant with the 2X line.
 53. A method for producing triggered pairs ofpolarization-entangled photons, comprising the steps of: (a) providing aphoton emitter; and (b) delivering two pulses in succession with a lightsource to said photon emitter, wherein first of said two pulses is tunedto a narrow absorption resonance to generate a first electron-hole pairin an excited state, and with a polarization of said light source chosento yield a definite spin, and second of said two pulses at a slightlylower energy is tuned to a narrow absorption resonance to generate asecond electron-hole pair in an excited state, and with a polarizationchosen to yield a spin opposite to that of said first electron-holepair.
 54. The method of claim 53, wherein said light source is a pulsedlaser.
 55. The method of claim 53, wherein said photon emitter is asemiconductor quantum dot.
 56. The method of claim 55, wherein saidsemiconductor quantum dot comprises a tiny island of smaller-bandgapsemiconductor material surrounded by a larger-bandgap matrix.
 57. Themethod of claim 56, wherein said semiconductor quantum dot is an InGaAsregion surrounded by GaAs, an InP region surrounded by GaInP, or a GaAsregion surrounded by AlGaAs.
 58. The method of claim 53, furthercomprising the step of filtering to reject scattered light from saidlight source.
 59. The method of claim 53, further comprising the step ofproviding a micro-cavity to increase the collection efficiency of saidemitted photons and to direct said emitted photons into a single spatialmode and coupling said single mode single photons to one or more opticalelements.
 60. The method of claim 59, wherein said micro-cavitycomprises a small volume and a means for long photon storage time. 61.The method of claim 59, wherein said micro-cavity is a micro-postdistributed-Bragg-reflector cavity or a micro-sphere cavity.
 62. Themethod of claim 59, wherein said micro-cavity has a first spatial modethat is resonant with the 1X line and a second spatial mode that isresonant with the 2X line.
 63. A device for producing triggered pairs ofpolarization-entangled photons, comprising: (a) a photon emitter; and(b) a light source to deliver a pulse to said photon emitter generatingseveral electron-hole pairs of both spins; and (c) a filter to isolateonly a single-exciton emission line and narrowly accept only theemissions line corresponding to two electron-hole pairs with oppositespins.
 64. The device of claim 63, wherein said light source is a pulsedlaser.
 65. The device of claim 63, wherein said photon emitter is asemiconductor quantum dot.
 66. The device of claim 65, wherein saidsemiconductor quantum dot comprises a tiny island of smaller-bandgapsemiconductor material surrounded by a larger-bandgap matrix.
 67. Thedevice of claim 66, wherein said semiconductor quantum dot is an InGaAsregion surrounded by GaAs, an InP region surrounded by GaInP, or a GaAsregion surrounded by AlGaAs.
 68. The device of claim 63, wherein saidfilter rejects scattered light from said light source.
 69. The device ofclaim 63, further comprising a micro-cavity to increase the collectionefficiency of said emitted photons and to direct said emitted photonsinto a single spatial mode and coupling said single mode single photonsto one or more optical elements.
 70. The device of claim 69, whereinsaid micro-cavity comprises a small volume and a means for long photonstorage time.
 71. The device of claim 69, wherein said micro-cavity is amicro-post distributed-Bragg-reflector cavity or a micro-sphere cavity.72. The device of claim 69, wherein said micro-cavity has a firstspatial mode that is resonant with the 1X line and a second spatial modethat is resonant with the 2X line.
 73. A method for producing triggeredpairs of polarization-entangled photons, comprising: (a) a photonemitter; (b) delivering a pulse with a light source to said photonemitter generating several electron-hole pairs of both spins; and (c)filtering with a filter only a single-exciton emission line and narrowlyaccept only the emissions line corresponding to two electron-hole pairswith opposite spins.
 74. The method of claim 73, wherein said lightsource is a pulsed laser.
 75. The method of claim 73, wherein saidphoton emitter is a semiconductor quantum dot.
 76. The method of claim75, wherein said semiconductor quantum dot comprises a tiny island ofsmaller-bandgap semiconductor material surrounded by a larger-bandgapmatrix.
 77. The method of claim 76, wherein said semiconductor quantumdot is an InGaAs region surrounded by GaAs, an InP region surrounded byGaInP, or a GaAs region surrounded by AlGaAs.
 78. The method of claim73, wherein said filter rejects scattered light from said light source.79. The method of claim 73, further comprising a micro-cavity toincrease the collection efficiency of said emitted photons and to directsaid emitted photons into a single spatial mode and coupling said singlemode single photons to one or more optical elements.
 80. The method ofclaim 79, wherein said micro-cavity comprises a small volume and a meansfor long photon storage time.
 81. The method of claim 79, wherein saidmicro-cavity is a micro-post distributed-Bragg-reflector cavity or amicro-sphere cavity.
 82. The method of claim 79, wherein saidmicro-cavity has a first spatial mode that is resonant with the 1X lineand a second spatial mode that is resonant with the 2X line.